You might try disabling crossfire, and re-running the benchmark. Crossfire is great for gaming, but might hurt your bench #'s for OpenCL. Although splitting the pci-E lane's down to 4 per GPU processor may also be whats slowing you down.

It's a common misconception that OpenCL requires the cards be in crossfire mode to get both working. This is only true for gaming, but not for applications that use a plugin like Neat. I know I tried it with crossfire on and my FPS went to crap as well.

The difference in single-GPU performance between 1-GPU and 2-GPU setups is likely caused by the fact that you are reducing the number of PCIe lanes allocated per each GPU. Neat Video pumps a lot of data via PCIe and any decrease in throughput will impact the overall performance.

Some Intel CPUs have 40 PCIe lanes which allows simultaneous transfers to two PCIe3 x16 GPUs at full speed. Ryzen CPUs seem to have up to 24 lanes, which is only enough for transferring data to one GPU at full speed (or even to both GPUs at reduced rates). Do you have any BIOS settings controlling the allocation of PCIe lanes to GPUs (perhaps you can do 16+8 instead of 8+8)?

Detecting the best combination of performance settings:
running the test data set on up to 16 CPU cores and on up to 2 GPUs
TITAN Xp COLLECTORS EDITION #1: 10169 MB currently available (12288 MB total), using up to 100%
TITAN Xp COLLECTORS EDITION #2: 10169 MB currently available (12288 MB total), using up to 100%

I just purchased NeatVideo and installed it and NeatBench on my new iMac Pro. I'm a bit surprised that the combination of CPU and GPU isn't markedly higher than either just the CPU or just the GPU. Is that normal, or a case of the hardware bing fairly new and drivers not yet fully mature?

Detecting the best combination of performance settings:
running the test data set on up to 20 CPU cores and on up to 1 GPU
AMD Radeon Pro Vega 64 Compute Engine: 16368 MB currently available, using up to 100%

The GPU drivers of these new models of GPUs coming with new iMacs are not very efficient as compared with the Windows versions of the drivers for the same GPUs. The same GPUs show better results in Windows according to our observations so far. We hope that the Mac versions of the drivers will be gradually improved by Apple.

Regarding the performance of the combination of CPU and GPU, that is not abnormal to see such an imperfect addition of speeds of two computing devices. I recommend to run the Optimize Settings test in Neat Video Preferences. The results shown there may be slightly different because the host application may work with somewhat different parameters (frame size, bitdepth, radius) and in those conditions the speeds may add up differently.

Thanks for the info, Vlad. I'm not surprised about the drivers being slightly slower, Apple tends to lean more toward stability rather than outright speed. I did run the optimization in Final Cut X, and the results were fairly close showing 15 cores+GPU to be optimum at 21.5fps.

Liking the results of NeatVideo a lot so far. It does wonders for VHS tapes converted to DVDs in the old stand alone machines. Converting and deinterlacing them in Handbrake and then into FCX with NeatVideo.

Detecting the best combination of performance settings:
running the test data set on up to 8 CPU cores and on up to 1 GPU
AMD Radeon Pro 555 Compute Engine: 2048 MB currently available, using up to 100%